i am just about to start the first floor of my rear extension. i have one question that i am concerned about.

i am fixing the brick and blockwork skins of the extension to the existing cavity wall using the usual wall starter kits. once the rear extension is built i will be knocking through the whole width of the existing rear of the house to create a larger bedroom.

in my opinion i need to put a vertical dpc inbetween the new cavity wall where the skins are attached to the existing brickwork.

however, by cutting a 3mm vertical strip and placing a vertical 100mm dpc this will weaken the part of the wall that the blockwork is attached to via wall started kit, therefore when i knock through, the blockwork will have the wall starter basically attached to a 100mm "pier".

i hope this makes sense, i know normally when putting an extension on the back of a house requires a vertical dpc but in this case it seems it would be weakening the wall and causing possibly more problems. this is a first floor bedroom extension and so i am not anticipating a huge amount of driving rain against the adjoining areas.

I'm not quite sure what you mean by an unsupported pier. The stability of the wall comes from the wall tie which tie the two skins together. If you do not have a sufficient masonry return then your structural engineer should be designing some type of pier, wind post.

The vertical dpc will only pass through the outer skin, inner skin will not be touched.

the unsupported pier comes from the fact that the wall starter kit is fixed to a cavity wall (the existing rear of the house), once i knock through there will be a stop end inside the new room and a disc cut line between the bricks that the wall starter is fixed to. i have been thinking about this and i think the best thing to do is to take out the existing rear brickwork to expose the existing block work internal skin at the rear of the house and attach my wall started kit to the back of the blockwork, this way when i knock thru i will also have a solid return on the pier??